When Blowfly Infestation's ability resolves, it will put a single -1/-1 counter on the targeted creature. It doesn't matter how many -1/-1 counters were on the creature that went to a graveyard as long as it had at least one on it.

5/1/2008

This ability is mandatory. If you're the only player who controls any creatures when this ability triggers, you must target one of them.